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Travel Time Reliability:
Making It There On Time, All The Time
This figure shows a line chart that represents a travel time frequency distribution for SR 520 Eastbound in Seattle during the evening peak period. The frequency distribution shows how many trips occur at different speeds. This line is roughly shaped like half of a mountain slope, with the peak of the mountain occurring at the free-flow travel time, the average travel time occurring mid-slope, and the 95th percentile travel time occurring at the bottom of the mountain. Calculations are shown for the planning time (which is equal to the 95th percentile travel time), the planning time index (the 95th percentile travel time divided by the free-flow travel time), and the buffer index (95th percentile minus the average travel time, divided by the average travel time).
Figure 5. Reliability measures are easy to calculate with continuous data
Source: Traffic Congestion and Reliability: Trends and Advanced Strategies for Congestion Mitigation, September 1, 2005, available at http://ops.fhwa.dot.gov/congestion_report/.
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